1. Field of the Invention
The present invention relates to an image forming apparatus for digitally correcting a distortion of an image accompanied by a curve or an inclination of a laser beam and to a control method for the image forming apparatus.
2. Description of the Related Art
In an image forming apparatus of an electrophotographic system, a method whereby a step of adjusting a laser scanner is reduced and a distortion of an image accompanied by a curve of a laser beam is digitally corrected to thereby reduce the costs has been disclosed in Japanese Patent No. 3388193.
For example, in a digital correction in a sub-scanning direction of a scan line, a line is properly changed based on a shift amount of the laser beam which has previously been obtained so that the shift amount can be set off, and the image is formed. The “line” mentioned here denotes a set of pixels arranged in a main scanning direction.
When describing in more detail, for example, when the shift amount of the laser beam from a position x in the main scanning direction is expressed by f(x), a number −y which is obtained from a value y obtained by rounding off f(x) is assumed to be a scan line changing amount. All data within an interval from xi to xj where the scan line changing amounts are equal is shifted by a distance corresponding to −y lines. By applying such a process to all image regions, the curve of the laser beam is set off and an original image can be reproduced.
Different from the above case, there is a case where a reference position of a sheet is deviated in the main scanning direction from an ideal position due to a tolerance or the like of a sheet conveying mechanism. In many cases, a deviation amount differs depending on a sheet feeding port.
A method of moving a writing position of an image based on a deviation amount of each sheet feeding port in order to correct those deviations has been known. With respect to the deviation amount of each sheet feeding port, there is an apparatus constructed in such a manner that it has been preset into a nonvolatile memory of the apparatus upon shipping from a factory, an apparatus constructed in such a manner that a user interface is provided and the user can properly change a set value as a correction amount, or the like.
However, in the related art, the following problem occurs in the case of simultaneously performing both of the digital correction in the sub-scanning direction of the scan line in the former case and the correction of the main scan positional deviation of each sheet feeding port in the latter case.
According to a correction amount s at which the writing position of the image has been moved in order to make the latter correction, the scan line changing amount in the former correction is calculated and a scan line changing amount can be calculated based on f(x+s) in place of f(x).
By calculating the scan line changing amount based on f(x+s), even if the writing position of the image is moved according to the deviation amount at the sheet feeding port, a color drift in the sub-scanning direction accompanied by the curve of the laser beam can be eliminated.
However, in the case of an image forming apparatus of what is called a host base in which a rendering of print image and the digital correction in the sub-scanning direction of the scan line are not executed in the image forming apparatus but are executed by a printer driver, there are the following problems.
That is, there is a case where the sheet feeding port cannot be specified at timing when the printer driver forms a print job. An apparatus in which even if the specific sheet feeding port is not designated, the sheet feeding port is automatically selected according to a sheet size or sheet type of the print job, or the like corresponds to such a case. In such an apparatus, when an absence of sheets has occurred at the sheet feeding port which was selected first, the sheet feeding port is switched to another sheet feeding port in which the sheets of the same sheet size and sheet type have been enclosed and the print is continued.
With respect to the correction of the main scan positional deviation of each sheet feeding port, a correction amount of the sheet feeding port which was selected first is assumed to be s1, a correction amount of the sheet feeding port which was selected second is assumed to be s2, and s1≠s2.
As described in the related art, it is demanded to calculate the scan line changing amount based on f(x+s) in place of f(x) according to the correction amount s of the writing position of the image.
It is now assumed that when the printer driver forms the print job, only the sheet feeding port which is selected first could be specified and the scan line changing amount was calculated based on f(x+s1). In this case, naturally, a print result to a sheet conveyed from the sheet feeding port which is selected first is good. However, if there are no sheets in the first sheet feeding port and the print is executed to a sheet conveyed from the sheet feeding port which was selected second, the scan line is deviated in the sub-scanning direction by a distance corresponding to f(x+s2)−f(x+s1).